Kale Pests and Diseases: Aphids, Caterpillars, Flea Beetle and Clubroot

Identify the damage before you treat anything

Holes in kale leaves have several possible authors and the right response differs for each. Ragged holes in the middle of leaves with green pellets nearby are caterpillars. A scatter of tiny round shot holes is flea beetle. Slime trails with irregular holes from the leaf margin inwards are slugs. Gray-white clusters packed into the growing point are cabbage aphids, and those do not make holes at all.

Check the undersides of leaves and the tight center of the rosette every time you pick. Nearly every kale pest hides there rather than on the exposed upper surface, and every one of them is trivial to deal with in the first week and a serious job by the third.

Fine mesh netting stops most of it before it starts

The single most effective thing you can do for kale is to cover the row with insect mesh from the day you plant. Cabbage white butterflies, cabbage moths and flea beetles all have to land on the plant to do their damage, and mesh with a hole size of about 1.3 mm stops all three. This is prevention that works, unlike almost every spray.

Two details decide whether it works. The mesh must be held clear of the foliage on hoops, because a butterfly will lay eggs straight through fabric resting on a leaf. And the edges must be buried or weighted continuously, since a single gap lets everything in and then traps it inside with your plants.

1. Put hoops in before you need them Set hoops tall enough that the mesh stays six inches (15 cm) above the tallest leaf even when the plants are full grown.
2. Bury or weight every edge Run soil, planks or sandbags along the whole perimeter. Check after wind, since a lifted corner undoes the entire exercise.
3. Inspect under the mesh weekly Lift it while you pick and check leaf undersides. Anything that got in before you covered will breed happily under protection.
4. Leave it on all season There is no pollination requirement for a leaf crop, so the mesh can stay until you pull the plants. Only lift it to pick and weed.

Mesh does not stop slugs, and it makes a warm sheltered place they enjoy. Clear old leaves and debris from under the cover whenever you lift it.

Cabbage aphids: get into the crown early

Cabbage aphids are gray-white, coated in a waxy bloom, and they pack tightly into the growing point where a casual glance will not find them. By the time a colony is visible from standing height it has usually been there three weeks and has distorted the newest leaves permanently. They also transmit virus, so numbers matter more than the direct feeding damage.

While a colony is small, a firm jet of water directed into the crown removes most of it, repeated every few days. A soap-based spray works if it actually contacts the insects, which means getting the spray down into the center and under the leaves rather than misting the top. Where a rosette is heavily infested, cut the whole rosette out and bag it: that is faster and more reliable than trying to clean it.

1. Check the crown at every picking Part the newest leaves at the center and look down into the growing point. That is where colonies start, not on the open outer leaves.
2. Jet them off with water Aim a firm jet into the crown. Most knocked-off aphids never make it back onto the plant. Repeat every three days until you find none.
3. Cut out heavily infested growth If the center is packed with them, cut that growth out and bag it. The plant will regrow from a lower bud far faster than you can clean it.
4. Ease off the nitrogen Overfed kale makes soft, sappy growth that aphids multiply on. If you have been feeding heavily, skip the next two feeds.

Caterpillars and flea beetle

Cabbage white caterpillars are the most visible kale pest and the easiest to handle if you look for the eggs rather than the damage. The butterflies lay yellow eggs in clusters on the undersides of leaves; wiping those off with a thumb takes seconds and prevents everything that follows. Cabbage moth caterpillars are sneakier, boring into the heart of the plant, so anything you find tunneling in the crown should be dug out.

Flea beetle is a different problem: tiny black beetles that jump when disturbed and pepper young leaves with round holes. Mature plants shrug it off, but seedlings can be killed outright. Beetles are worst on dry, dusty soil, so keeping the surface consistently damp and mulched cuts the damage substantially, and mesh over the seedbed removes it entirely.

1. Wipe off egg clusters weekly Turn leaves over and look for clusters of small yellow eggs. Rub them off with a thumb. One minute a week replaces all later caterpillar work.
2. Pick caterpillars in the evening They feed at dusk and sit exposed. Check the leaf undersides and the crown with a torch and remove what you find by hand.
3. Use a biological spray for a heavy attack A Bacillus thuringiensis product approved for edible crops where you live works only on caterpillars and only if they eat it, so spray both leaf surfaces and repeat after rain.
4. Keep seedbeds damp against flea beetle Water the surface daily while plants are small and mulch once they are established. Dry dusty soil is what flea beetle populations build on.

Clubroot and downy mildew: soil and air problems

Clubroot is the one kale problem with no cure. It is a soil-borne organism that swells the roots into distorted clubs so the plant cannot take up water; the symptom above ground is wilting in sunshine and recovery overnight, then stunting and death. Its spores survive in soil for up to twenty years, so the whole strategy is avoidance: never plant brassicas in the same ground more than one year in four, lift and burn affected plants with their soil, and raise the soil pH to about 7.0 with lime, which suppresses the organism considerably.

Downy mildew is far less serious. It shows as yellow blotches on the upper leaf surface with a fine gray-purple felt underneath, and it thrives in cool, damp, still air on crowded plants. Space plants properly, strip the lower leaves to let air through, water the soil rather than the foliage, and remove affected leaves as soon as you see them. It rarely kills a kale, it just costs you leaves.

Write down where each brassica row ran and the year. A four year rotation is impossible to keep honest from memory once a bed has been dug over, and clubroot is the mistake you cannot undo.